Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of undesired vapor emissions that could release fuel vapors to the atmosphere. Undesired vapor emissions may be identified using engine-off-natural-vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system and evaporative emissions control system may be isolated at an engine-off event. The pressure in such a fuel system may increase during a pressure phase portion of the EONV test if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. A pressure rise above a threshold may indicate an absence of undesired fuel system vapor emissions. Alternatively, in the absence of a pressure rise above a threshold, as a fuel system cools down, a vacuum may be generated therein during a vacuum phase of the EONV test as fuel vapors condense to liquid fuel. Vacuum generation may be monitored and undesired vapor emissions identified based on expected vacuum development or expected rates of vacuum development.
Entry conditions and thresholds for an EONV test may be based on an inferred total amount of heat rejected into the fuel tank during the prior drive cycle. The inferred amount of heat may be based on engine run-time, integrated mass air flow, fuel level, ambient temperature, reid vapor pressure, etc. While these heat rejection inferences work well in most conditions, they may be prone to errors when noise factors are involved. For example, if a vehicle is driven downhill for an extended period, driven under rainy and/or windy conditions, or under conditions where a period of high-speed driving is followed by a period of idling, much of the heat rejection to the fuel tank may be negated. As a result, in an example where an EONV test is executed based on a heat rejection inference where the above-described noise factors are involved may result in a false failure. Furthermore, relying solely on heat rejection for conducting EONV diagnostics may be problematic for hybrid vehicles, where engine run-time may be limited, thus limiting an amount of heat rejected from the engine for particular drive cycles.
Still further, a typical EONV test may be enabled to run for a predetermined time duration (e.g. 45 minutes), where the time limit may be a function of battery power. Accordingly, if a vehicle initiates an EONV test at a vehicle-off event, and the vehicle does not pass during the pressure phase of the EONV test, then the time spent conducting the pressure phase decreases an amount of time for the vacuum phase portion of the EONV test to be conducted. If the vehicle does not then pass during the vacuum phase portion within the allotted predetermined time duration (e.g. 45 minutes), then the presence of undesired evaporative emissions may be falsely indicated in a case where the fuel system and evaporative emissions system are free from undesired evaporative emissions. In addition to the potential false indication of the presence of undesired evaporative emissions for an EONV test that did not pass on the pressure phase, and where the predetermined time duration expired prior to the EONV test passing on the vacuum phase, such a test wastes battery power, which may negatively impact fuel economy. Still further, such EONV tests that are initiated, but where the time limit expires prior to indicating a passing result, may additionally result in increased loading of a fuel vapor canister, increased wear and tear on valves that are actuated open or closed to conduct the EONV test, etc. Thus, a more intelligent means of determining when and how to execute diagnostic tests for undesired evaporative emissions, is desired.
The inventors have herein recognized the above-mentioned issues, and have developed systems and methods to at least partially address them. In one example, a method is provided, comprising setting an initial vent duration for an engine-off-natural-vacuum (EONV) test as a function of a likelihood that a vehicle will pass the EONV test during a pressure phase portion, or during a vacuum phase portion, and commencing the EONV test with the vacuum phase portion responsive to the likelihood the vehicle will pass during the vacuum phase portion. In this way, fuel economy may be improved and battery power may be conserved by avoiding the pressure phase portion if it is not likely to succeed or pass the EONV test.
As an example, the method may include commencing the EONV test with the vacuum phase portion and not conducting the pressure phase portion, regardless of whether the vehicle passes the vacuum phase portion or not.
As another example, responsive to the likelihood that the vehicle will pass the EONV test during the pressure phase portion, commencing the EONV test with the pressure phase portion, and then subsequently conducting the vacuum phase portion responsive to the vehicle not passing during the pressure phase portion.
As another example, passing the EONV test during the pressure phase portion may comprise pressure in a fuel system and evaporative emissions system of the vehicle reaching or exceeding a negative pressure threshold. In some examples, the fuel system and evaporative emissions system may be sealed from atmosphere during the pressure phase portion and vacuum phase portion of the EONV test.
Another examples includes where the initial vent duration is shorter given the likelihood that the vehicle will pass the EONV test during the pressure phase portion, as compared to the likelihood that the vehicle will pass during the vacuum phase portion. As an example, the initial vent duration may comprise 30-60 seconds given the likelihood that the vehicle will pass on the pressure phase portion, and where the initial vent duration may comprise greater than 30-60 seconds given the likelihood that the vehicle will pass on the vacuum phase portion. In some examples, the initial vent duration may be variable given the likelihood that the vehicle will pass on the vacuum phase portion.
As another example, the likelihood that the vehicle will pass during the pressure phase portion or during the vacuum phase portion includes retrieving a set of most recent EONV test results from a plurality of vehicles of a similar class as the vehicle, within a threshold radius of the vehicle, and responsive to an indication that the plurality of vehicles are tending to not pass the pressure phase portion of the engine-off-natural-vacuum test, commencing the EONV test with the vacuum phase portion of the EONV test. For example, retrieving the set of most recent EONV test results further comprises indicating that the set of most recent EONV results correspond to tests conducted subsequent to similar drive cycle and environmental conditions as a current drive cycle of the vehicle.
In another example, the likelihood that the vehicle will pass during the pressure phase portion or the vacuum phase portion further comprises retrieving current and forecast weather conditions just prior to conducting the engine-off-natural-vacuum test, and indicating whether weather conditions support the vehicle passing during the pressure phase portion or during the vacuum phase portion.
In still another example, the likelihood that the vehicle will pass during the pressure phase portion or during the vacuum phase portion is a function of learned driving routes and associated engine-off-natural-vacuum test results.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.